Aetae,
что-то не так ...
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
.item {
border: 1px solid #008000;
margin: 4px;
}
.active {
border: 1px solid red;
}
</style>
<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/3.2.37/vue.runtime.global.min.js" ></script>
</head>
<body>
<div id="app">
<div v-for="{id} in itemsData" :class="[{ active: itemsFlag[id] },'item']">
<button @click="$set(itemsFlag, id, !itemsFlag[id])">Клик</button>
</div>
</div>
<script>
const app = new Vue({
el: "#app",
data: {
itemsData: [{
id: "a"
}, {
id: "b"
}, {
id: "c"
}],
itemsFlag : {
}
}
});
</script>
</body>
</html>